The subject matter disclosed herein relates to a system and method of acquiring three-dimensional coordinates of points on a surface of an object and in particular to a system and method of operating a laser tracker in conjunction with a scanner device to track the position and orientation of the scanner device during operation.
The acquisition of three-dimensional coordinates of an object or an environment is known. Various techniques may be used, such as time-of-flight (TOF) or triangulation methods for example. A TOF system such as a laser tracker, for example, directs a beam of light such as a laser beam toward a retroreflector target positioned over a spot to be measured. An absolute distance meter (ADM) is used to determine the distance from the distance meter to the retroreflector based on length of time it takes the light to travel to the spot and return. By moving the retroreflector target over the surface of the object, the coordinates of the object surface may be ascertained. Another example of a TOF system is a laser scanner that measures a distance to a spot on a diffuse surface with an ADM that measures the time for the light to travel to the spot and return. TOF systems have advantages in being accurate, but in some cases may be slower than systems that project a plurality of light spots onto the surface at each instant in time.
In contrast, a triangulation system such as a scanner projects either a line of light (e.g. from a laser line probe) or a pattern of light (e.g. from a structured light) onto the surface. In this system, a camera is coupled to a projector in a fixed mechanical relationship. The light/pattern emitted from the projector is reflected off of the surface and detected by the camera. Since the camera and projector are arranged in a fixed relationship, the distance to the object may be determined from captured images using trigonometric principles. Triangulation systems provide advantages in quickly acquiring coordinate data over large areas.
In some systems, during the scanning process, the scanner acquires, at different times, a series of images of the patterns of light formed on the object surface. These multiple images are then registered relative to each other so that the position and orientation of each image relative to the other images is known. Where the scanner is handheld, various techniques have been used to register the images. One common technique uses features in the images to match overlapping areas of adjacent image frames. This technique works well when the object being measured has many features relative to the field of view of the scanner. However, if the object contains a relatively large flat or curved surface, the images may not properly register relative to each other.
Accordingly, while existing coordinate measurement devices are suitable for their intended purposes, the need for improvement remains, particularly in improving the registration of images acquired by a scanner device.